Fastest Milky Way Star S301 Orbits Supermassive Black Hole

Astronomers have identified the fastest known star in the Milky Way galaxy, designated S301, as it hurtles around the supermassive black hole at the galactic center. This remarkable star travels at approximately 15,500 miles per second, a speed over 100,000 times faster than that of a commercial airplane. S301 is also the closest known star to the Milky Way’s central black hole, known as Sagittarius A* (Sgr A*), maintaining an orbit as close as Saturn is to our Sun.

"What is special about this star is that it’s orbiting Sagittarius A* on a very tight orbit, taking just 8.7 years to complete it, and is approaching the black hole at a mere 12 times the distance of Earth to the sun. That is unprecedented," stated Felix Mang, a PhD student at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, and a lead author of the study detailing the findings. The star's extreme proximity to Sgr A* offers a unique opportunity to study the black hole’s fundamental properties and the intense environment that surrounds it.

According to Einstein's general theory of relativity, massive objects like black holes, especially those that spin, warp the fabric of spacetime around them. This warping significantly influences the orbits of nearby celestial bodies. While this effect is most pronounced for objects orbiting fast-rotating black holes at very close range, S301's orbit exhibits the ideal combination of being both highly eccentric and exceptionally tight, making it a prime candidate for observing these relativistic effects.

Probing Black Hole Spin

"For the first time, we would actually be able to measure very directly the spin of a massive black hole, which would be a key test of Einstein’s theory," explained Dr. Stefan Gillessen, a senior scientist at MPE and a co-author of the research, published in the journal Nature. Previously, estimating the spin of Sgr A* was thought to require decades of observations tracking multiple stars. However, the team was able to trace S301’s orbit back to 2017. By monitoring its next closest approach to Sgr A* in 2031, scientists anticipate having sufficient data to determine the black hole's spin.

"With this star we hope to measure, within the next 10 years, the spin of the black hole," Mang added. The groundbreaking observations were conducted using the European Southern Observatory’s Very Large Telescope Interferometer (ESO’s VLTI) at the Paranal Observatory in Chile. This powerful instrument combines light from four 8-meter telescopes to achieve highly detailed measurements. Despite its perilous proximity to the central black hole, S301 is not expected to cross the event horizon, the point of no return.

"Black holes are perceived as these cosmic vacuum cleaners engulfing everything they can get a handle on," Mang elaborated. "But it’s just a massive object. It’s not really much different from something orbiting the sun and it’s going to remain on this orbit." Dr. Ziri Younsi, an astrophysicist at University College London who was not involved in the study, commented on the significance of the discovery: "This is incredibly important as a discovery. It’s very exciting because this thing comes insanely close to Sagittarius A* and it’s travelling, at its peak, at about 8% of the speed of light, which is crazy." Younsi further noted that using S301's orbit to accurately measure Sgr A*'s spin could revolutionize our understanding of the Milky Way's evolution and the fundamental physics governing black holes.